The present invention relates to an etching method and an etching apparatus, and particularly to an etching method capable of performing an etching with high selectivity and high anisotropy using a high density plasma under a low gas pressure, and an etching apparatus for carrying out the etching method.
In recent years, as the integration density of a semiconductor integrated circuit has been enhanced, the dimensions of patterns formed by etching in the manufacturing process have been further minimized. As the dimension of a pattern formed by etching is minimized, the aspect ratio of the pattern is inevitably increased, and accordingly the aspect ratio dependency of the etching rate is at stake. Namely, along with the increased aspect ratio, it has been difficult to make larger the etching rate.
An etching process involves irradiating energetic ions (hereinafter, referred to as etching ions, or simply as ions) for etching on the surface of an article to be etched absorbed with an etchant; forming reaction products between the etchant and surface components of the article to be etched by impact of the ions; and desorbing the reaction products from the surface of the article to be etched; thereby obtaining the desired pattern.
In this case, only ions accelerated by an electric field are perpendicularly incident on the surface of an article to be etched, and an etchant is incident on the surface of the article to be etched from the random directions. Accordingly, to reduce the aspect ratio dependency of the etching rate, it is required that etching ions are perpendicularly incident on the surface of the article to be etched. In this regard, various techniques have been proposed. To arrange the advancing directions of the etching ions for allowing the etching ions to be perpendicularly on the surface of the article to be etched, there is known a method of reducing the gas pressure of plasma. This method is intended to improve the advancing directions of the etching ions by reducing the scattering of the etching ions through the collision between the etching ions and neutral particles in an ion sheath. Further, in this case, since the densities of the etchant and the etching ions are lowered when the gas pressure is reduced, the etching rate is inevitably reduced. For preventing the reduction in such an etching rate, there has been proposed a technique of applying a magnetic field in plasma for forming a high density plasma.
Additionally, in formation of an ultra large scale integrated circuit (ULSI), the thinning of a mask and a bottom layer has been advanced, and the etching technique excellent in high selectivity and in low damage has been required. To meet the requirements, the lowering of the energy of etching ions has been performed.
As the etching apparatus of performing the etching under a low gas pressure and a low ion energy for satisfying both requirements, an etching apparatus using microwave discharge [called ECR (electron cyclotron resonance) etching apparatus] has been put in practice, and further, its improvement has been advanced. For example, Japanese Patent Laid-open No. HEI 2-253617 discloses an etching apparatus, wherein a DC current superimposed with an AC current is allowed to flow in an electric magnet disposed around a reactor for improving the plasma uniformity, thereby enhancing the etching rate. Further, Japanese Patent Laid-open No. HEI 3-155620 discloses an apparatus, which is intended to intermittently generate a plasma by switching a microwave power supply for generating a plasma synchronously with a DC bias power supply for accelerating ions. In this apparatus, the matching of the DC bias is effectively achieved, to reduce the loss of the bias power, thus enhancing the etching rate.
In the above-described prior art etching methods and apparatuses, for improving the directional uniformity of etching ions and for lowering the energy, a high density plasma has been generated under a low gas pressure, and the magnitude of a radio frequency bias power for accelerating ions has been reduced. These prior art techniques has given consideration in reducing the scattering of ions assisting etching for improving the directional uniformity of ions; however, these methods and apparatuses have not given sufficient consideration in improving the uniformity of ions in an accelerated electric field and in supplying an etchant in a sufficient amount. Accordingly, for example, in formation of trench or hole patterns by etching, there occurs such a disadvantage that the supply of an etchant to the bottom portion of a trench or a hole is insufficient, and thus, although etching ions are approximately perpendicularly incident on the surface of an article to be etched, the etching rate at the portion with a high aspect ratio is lowered, thus making it impossible to obtain the sufficient depth of etching. As the etching dimension has been minimized, the aspect ratio has been further enlarged. Consequently, the above problem has become more and more important.
In addition, there occurs such an inconvenient phenomenon that etching ions are curved by a local electric field strain such as the charge-up of a mask on the article to be etched, and the pattern thus obtained becomes oblique. The phenomenon of curving ions is generated by the concentration of an electric field due to pattern steps present on the surface of an article to be etched. As a high density plasma is formed for increasing an etching rate, the thickness of an ion sheath is made thin relatively to the dimension of the step of the pattern formed by etching, ions are more largely curved. As a result, the above phenomenon has become furthermore important.